1. Field of the Invention
The invention relates generally to methods and systems for optical inspection and measurement techniques, and more particularly to methods and systems for inspecting and measuring surface and/or volume characteristics of transmissive and/or reflective optical articles.
2. Description of the Related Art
Many optical devices and systems include optical articles having desired optical characteristics and tolerances relating to, e.g., surface flatness and defect characteristics such as divots or peaks on optical surfaces as well as refractive index values of transmissive articles. The performance of an optical article is typically highly dependent on the quality of the optical surfaces and index values of the article. For example, optically flat surfaces may have desired tolerances relating to the smoothness and relative size of defects on the surface, where variations from the desired tolerances degrade the article performance. Both fine scale smoothness, and coarse scale deviations from the design target are factors of the article performance. Additionally, performance of transmissive optical articles is generally dependent on index homogeneity within the article (or in the case of a gradient index, or “GRIN,” element on the error between the intended index values versus the actual index values). Accordingly, it is desirable to inspect optical surface characteristics and volume characteristics of optical articles to determine if they are within desired tolerances for a particular use or application.
Optical articles may be inspected for small and large scale defects in many ways. Most commonly, an interferometer is used to test the wavefront emerging from a surface of an optical article. A variety of commercially available interferometer systems are used for this purpose. Exemplary systems commonly include Fizeau or Twyman-Green type interferometers that measure the distortion of surfaces or transmitted wavefronts as is known in the art. Such systems do not include any physical contact with the optical article that is under inspection and are particularly useful for testing or inspecting surfaces of computer disks, silicon wafers/structures, and the like.
Generally, interferometer systems include a laser that produces a coherent light beam which is expanded by the combination of a focusing lens and a collimator lens into a collimated light beam. The collimated light beam is then divided into two beams of equal intensity, e.g., by a 50% reflecting beam splitter in the path of the collimated light beam. A first beam reflects from the surface of the optical article under inspection and a second beam reflects from a reference surface. The beam transmitted through or reflected from the article is the test beam and the beam reflected from the reference surface serves as a reference beam. The test beam and the reference beam are combined and directed together toward an imaging lens and a sensor. The sensor detects the two beams and interference fringes associated with variations in the surface of the optical article under inspection relative to the reference surface. In particular, the interference patterns may be analyzed to provide information regarding the smoothness of the surface and defect features of the surface such as grooves, divots, peaks, and the like.
Interferometer systems and methods have several disadvantages for inspecting the optical quality of articles. For example, the interferometer system includes relatively expensive equipment, and generally must be dimensionally stable to less than the wavelength of the light being used in order to accurately observe the interference fringes. Additionally, interferometer methods compare the optical article under inspection to a reference object/surface such that inspection accuracy depends in large part on the reference object. The reference object is therefore generally made to high precision standards, often at substantial cost. Also, fine-pitched features (e.g., surface roughness) are generally not well resolved with interferometer methods and systems. Finally, interferometer methods generally do not allow for inspection or measurements of characteristics of the volume of the optical article, e.g., to determine index homogeneity or other characteristics at image planes within one or more layers of a transmissive and/or reflective optical article.